Stomata from npq1, a zeaxanthin-less arabidopsis mutant, lack a specific response to blue light

The Arabidopsis mutant npq1, which cannot accumulate zeaxanthin because of a defective violaxanthin de-epoxidase, was used to investigate the role of zeaxanthin in the stomatal response to blue light, Neither dark-adapted nor light-treated guard cells or mesophyll cells of the npq1 mutant contained detectable zeaxanthin, In contrast, wildtype guard cells had a significant zeaxanthin content in the dark and accumulated large amounts of zeaxanthin when illuminated. The well-documented red light enhancement of blue light-stimulated stomatal opening, in which increasing fluence rates of background red light result in increased response to blue light, was used to probe the specific blue light response of Arabidopsis stomata, Stomata from the npq1 mutant did not have a specific blue light response under all fluence rates of background red light tested, On the other hand, stomata from leaves of hy4 (cry 1), an Arabidopsis mutant lacking blue light-dependent inhibition of hypocotyl elongation, had a typical enhancement of the blue light response by background red light. The lack of a specific blue light response in the zeaxanthinless npq1 mutant provides genetic evidence for the role of zeaxanthin as a blue light photoreceptor in guard cells.